A known image forming apparatus has a function to correct deviation etc. in an image forming position on, for example, a sheet. More specifically, the image forming apparatus forms a plurality of marks (that configure a pattern such as a registration pattern) on a belt and, while emitting light toward the belt, receives the reflection light using an optical sensor. Then, on a basis of a light reception signal outputted from the optical sensor, the image forming apparatus reads a difference between a reflectance (a quantity of reflected light) of a belt surface and a reflectance of a mark surface to determine the mark position on the belt. On a basis of a result of the determination, the apparatus corrects the deviation in the image forming position.
Note here that, the belt surface can get scratches and waste, and the scratches and waste can diffuse the reflection light. This causes decrease in the reflectance of the belt surface and can result in failure in determination of the mark position. Note that there is a known art addressed to reduce this failure. With the art, the apparatus evaluates a condition of the reflection of the belt surface and, according to a result of the evaluation, adjusts the sensitivity of the optical sensor.
Note that the degree of the scratches and waste varies by position on the belt surface and, accordingly, the reflectance of the belt surface varies by position; following this, the light reception signal also varies. However, the known art is configured to evaluate the condition of the reflection of the belt surface on the basis of the light reception signal at a single time point, i.e. without considering the reflectance variation. Therefore, even with the adjusted sensitivity of the optical sensor, accuracy in determining the mark position varies depending on the level of the light reception signal of the single time.
Note that the variation in the light reception signal can be caused not only by the variation in reflectance of the belt surface; the variation in the light reception signal can be caused also by other factors such as variation in quantity of light emitted from the optical sensor, in the photoelectric conversion efficiency of the optical sensor, etc.